Historically, many individuals have required the use of pressure support ventilators in order to assist with respiratory problems, such as sleep apnea. Thus, over the years, a wide range of pressure support ventilators have been developed and produced that are designed to meet the needs of the individuals in question.
Essentially, the primary objective of a pressure support ventilator is to impart to a respiratory patient sufficient air (or gas) pressure in his or her airway so as to either preliminarily obviate the effect of any resistances or impedances within the patient's airway that might otherwise arise in the context of a patient's normal breathing patterns (e.g. sleep apnea) or even overcome airway resistances or impedances that might be of a more static nature (e.g. longer-term constrictions of the airway, or even lungs, caused by any of a number of possible physiological factors). As briefly discussed below, this impartation of pressure to a patient's airway has taken different forms over the years.
One basic form of pressure support that was developed initially was a so-called "continuous positive airway pressure", or "CPAP" support system, including those systems developed by Respironics, Inc., of Murrysville, Pa. This would involve the provision essentially of one constant pressure throughout a patient's entire breathing pattern. Although the single constant pressure was normally sufficient to overcome a patient's airway resistances or impedances (as discussed above), it was often found that a patient's ability to effectively exhale in such a context might be difficult.
An innovation over the above-described "CPAP", as developed by Respironics, Inc., was what was regarded as a "bi-level" support system and is implemented in the Respironics BiPAP.RTM. devices. This involved the provision of suitable sensing components for detecting the periods of time during which a patient would either inhale or exhale. Consequently, the sensing components were utilized in such a way that, during inhalation, one given pressure could be provided to the patient, while during exhalation, another, notably reduced, pressure could be provided. Several U.S. patents describe this "bi-level" system in detail, including U.S. Pat. Nos. 5,433,193; 5,313,937; 5,239,995; and 5,148,802, all of which are hereby expressly incorporated by reference as if set forth in its entirety herein.
In further innovations, apparatus and methods were developed in which the air or gas pressure supplied to a patient in the context of a pressure support system could be variable, either depending on a preprogrammed algorithm that exacted a predetermined time-dependent pattern of pressures on the patient or specific, instantaneously monitored demands of the patient. A device along these lines is disclosed in U.S. Pat. No. 5,535,738, which is also hereby expressly incorporated by reference as if set forth in its entirety herein.
Generally, whether the pressure administered to a patient is intended to be virtually constant over given periods of time (as in a "BiPAP" system) or variable, there has typically been a need to monitor the pressure being provided to the patient with a feedback control system so that, if necessary, the pressure can be readjusted so as to conform to the desired output pressure. Typically, such feedback control has not involved adjustment of the blower motor, as there are logistical problems associated with this (such as imprecisions in changing the motor speed within a short period of time, for instance). Accordingly, many conventional systems have included a single pressure relief valve, situated between the blower outlet and the start of the "patient circuit" (i.e. that portion of the respiratory circuit that includes tubing, filters, water traps, valves, etc. that are regarded as being necessary for providing suitably conditioned air or gas to the patient in question), that serves to readjust the air or gas pressure in the respirator circuit in response to prompts from the feedback control system. Such a pressure relief valve would also be typically used for exhausting air and/or gas emanating from a patient upon exhaling (thereby causing a net "negative" flow in the respirator circuit, i.e. back towards the blower, which would be vented off through the pressure relief valve).
Because such pressure relief valves are typically embodied by proportional valves that open to merely vent a portion of the flow of the respirator circuit to the ambient atmosphere (or other predetermined remote location), it is essentially the case that they are only able to increase pressure within the system upon being further closed from an already open position, wherein a fully "open" position would result in maximum venting from the system (thereby resulting in decreased pressure in the patient circuit) and a fully "closed" position would result in minimal or nonexistent venting from the system (thereby resulting in increased pressure in the patient circuit).
Because, especially in the context of respiratory patients who may require increased airway pressure from time to time, it must often be assumed that an allowance must be made for increasing the pressure provided to the patient from a lower level, it was therefore often necessary, in the context of systems utilizing a pressure relief valve as discussed above, to keep the valve open during normal inhalations and exhalations of the patient, with the understanding that a sudden, instantaneous demand for greater pressure (as initiated, for example, by an unexpectedly occurring blockage or constriction in the patient's airway and as measured by appropriate pressure and/or flow sensors at the patient circuit), could be attended to by closing the pressure relief valve to the degree sufficient for providing the demanded pressure. Alternatively, and especially in the context of more continuous systems such as the "BiPAP", it would often merely be the case that a wide range of constant inhalation pressures (as provided by the system) had to be allowed for in manufacturing the systems in question, in view of their use among a wide variety of patients with widely varying static pressure requirements. In any case, in order to assure a sufficient margin of safety, there was often the consequence that tremendous levels of air or gas flow would be vented away by the pressure relief valve, to be "reined in" only in those instances where greater pressure (for the patient) would be required.
Thus, although the use of a single pressure relief valve, as discussed above, can offer stable, fast open-loop pressure control, its utility has been limited by the level of pressure and flow required from the blower fan, which itself has not been, and essentially cannot be, used for pressure control. High motor speeds in this context have the tendency of increasing bearing and motor loads (in proportion to the square of motor speed), while the resultant excessive flow has the tendency of wasting power since work needs to be done to the fluid to bring it up to pressure, only to release it to the atmosphere. It stands to note that the exhausting of large gas flows also tends to be very noisy.
Recent years have also seen the development of, and demands for, blower-based pressure support ventilators that provide considerably larger pressures than conventional devices. Since this would normally result in the provision of ventilators having blowers that would subsequently provide astronomically excessive levels of flow that, in the context of single pressure-relief valve arrangements as discussed above, would result in tremendous wasted flow and unprecedentedly excessive noise levels (from exhaust), a need has arisen to efficiently regulate respirator flow and pressure in a manner that minimizes waste while still providing the patient with adequate respiratory support.
Another perennial challenge in the context of pressure-support ventilators is found in that historically, many individuals, including patients with given types of lung disease, have required not only general pressure-support respiratory assistance but also additional assistance in the form of elevated concentrations of oxygen, with the objective of maintaining proper levels of oxygen in the arteries. Consequently, the pressure-support ventilators that have been developed over the years, such as those discussed in the U.S. patents cited further above, have concomitantly placed demands on the manner in which arrangements for providing supplemental oxygen can be incorporated into the ventilators.
To this end, various methods of "oxygen mixing" have been developed with varying degrees of success. In at least one known realization, involving blower-based, non-invasive ventilators, supplemental oxygen is effectively "bled" into the individual's airstream in the immediate vicinity of the mask worn by the individual being treated. However, such known arrangements have tended to utilize continuous oxygen flows through the mask, with the undesirable result that a considerable quantity of oxygen may be wasted and continues to flow whether the individual is inhaling or exhaling. Additionally, the concentration of oxygen in the inspired air is unknown, thus inhibiting the ability of a clinician to control such treatment. Further, there is a concomitant danger in the possible effects of inadvertently venting oxygen directly into the atmosphere, either via excessive oxygen flow that leaves the vicinity of the individual's mask or through other leaks in the system, or even into the interior of a respirator apparatus, where the vented or leaked oxygen might interact with high temperatures within the unit and invite the risk of combustion.
A need has therefore arisen in conjunction with the provision of an oxygen mixer that accurately provides individuals with a known and accurately predetermined quantity of supplemental oxygen in a ventilator device such as a pressure support ventilator.